Field of the Invention
The present invention relates generally to nuclear fuel assemblies and, more particularly, is concerned with a nuclear fuel rod grid having unique fuel rod engaging spring and dimple structures.
2. Descriotion of the Prior Art
In most nuclear reactors, the reactor core is comprised of a large number of elongated fuel assemblies. Conventional designs of these fuel assemblies include a plurality of fuel rods held in an organized array by a plurality of grids spaced axially along the fuel assembly length and attached to a plurality of elongated control guide thimbles of the fuel assembly. Top and bottom nozzles on opposite ends of the fuel assembly are secured to the guide thimbles which extend slightly above and below the ends of the fuel rods.
The grids as well known in the art are used to precisely maintain the spacing between the fuel rods in the reactor core, prevent rod vibration, provide lateral support for the fuel rods, and, to some extent, frictionally retain the rods against longitudinal movement. Representative grid designs include those of some of the patent applications cross-referenced above.
One popular conventional grid design, being illustrated and described in U.S. Pat. Ser. No. 4,492,844 to Kobuck et al and assigned to the assignee of the present invention, includes a multiplicity of interleaved inner and outer straps having an egg-crate configuration foming a multiplicity of cells which individually accept the fuel rods and control rod guide thimbles. The cells of each grid which accept and support the fuel rods at a given axial location therealong typically use relatively resilient springs and/or relatively rigid protrusions (called dimples) formed into the metal of the interleaved straps. The springs and dimples of each grid cell frictionally engage or contact the respective fuel rod extending through the cell. Additionally, the outer straps are attached together and peripherally enclose the inner straps to impart strength and rigidity to the grid.
However, several drawbacks arise from this particular grid design. The vertical orientation and configuration of the fuel rod engaging springs require that the springs and thus the grid be of substantial height in order for the springs to have the desired amount of resiliency to function properly. This requirement increases the amount of parasitic structural material utilized in the fuel assembly and the pressure drop through the fuel assembly. Also, the horizontal orientation of the grid dimples, although providing for coolant flow through the dimples without an accompanying pressure drop, causes catching or hang-up of the fuel rods on the dimples as the fuel rods are pulled through the grid. On the other hand, dimples of vertical orientation, although solving the fuel rod hang-up problem, don't allow for coolant flow through the dimples and thus increase pressure drop.
Representative of other prior art grid designs are the ones described and illustrated in U.S. Pat. Nos. to Kooistra (3,070,534), Warberg (3,679,547), Jabsen (3,795,040), Wachter et al (3,928,131), Piepers et al (3,646,994), Creagan et al (4,061,536) and Masetti (4,090,918); French Pat. No. 1,497,083; West German Pat. No. 1,961,035; and Japanese Pat. No. 61-90085. While all of these grids may function satisfactory and generally achieve the objectives for which they were designed, none would appear to suggest an approach which will satisfactorily overcome the aforementioned drawbacks of the one conventional grid design briefly described above. Consequently, a need still exists for an improved grid spring structure which will avoid the above drawbacks without presenting new ones in their place.